1. Field of Invention
The present invention relates to an exhaust gas recirculation system for an internal combustion engine that recirculates exhaust gas to the upstream side of a compressor of a turbocharger.
2. Description of Related Art
Japanese Patent Application Publication No. 2002-106398 (hereinafter, referred to as “JP-A-2002-106398”) describes a conventional exhaust emission control system for an internal combustion engine equipped with a supercharger in which an inlet of recirculated exhaust gas is provided on the upstream side of the compressor of a turbocharger, and a throttle valve is arranged on the upstream side of this inlet. The exhaust emission control system includes a failsafe means for feeding air at a predetermined flow rate to the compressor to prevent the pressure upstream of the compressor from becoming excessively negative when a malfunction causes the throttle valve to become fully closed. In addition, a technique related to the present invention is described in Japanese Utility Model Application Publication No. 62-067926 (JP-U-62-067926).
In an internal combustion engine in which a portion of the exhaust gas is recirculated from an exhaust passage to an intake passage upstream of the compressor of a turbocharger and downstream of a throttle valve, the pressure in the intake passage between the throttle valve and the compressor (hereinafter, this pressure simply referred to as “the intake pressure upstream of the compressor”) is varied to vary the flow rate of the exhaust gas to be recirculated. For example, to increase the flow rate of the recirculated exhaust gas, the intake pressure upstream of the compressor is reduced to increase the difference between the pressure in the exhaust passage and the pressure in the intake passage. Because the system described in JP-A-2002-106398 does not take the recirculated exhaust gas into consideration when controlling the intake pressure upstream of the compressor, the intake pressure upstream of the compressor may become excessively negative when increasing the flow rate of recirculated exhaust gas. In this case, for example, the intake pressure in the compressor housing may drop excessively, which may, in turn, cause oil to leak from the oil seal portion of the bearing of the compressor.